The barley MLA13-AVRA13 heterodimer reveals principles for immunoreceptor recognition of RNase-like powdery mildew effectors.

Co-evolution between cereals and pathogenic grass powdery mildew fungi is exemplified by sequence diversification of an allelic series of barley resistance genes encoding Mildew Locus A (MLA) nucleotide-binding leucine-rich repeat (NLR) immunoreceptors with an N-terminal coiled-coil domain (CNLs). Each immunoreceptor recognises a matching, strain-specific powdery mildew effector encoded by an avirulence gene (AVRa). We present here the cryo-EM structure of barley MLA13 in complex with its cognate effector AVRA13-1. The effector adopts an RNase-like fold when bound to MLA13 in planta, similar to crystal structures of other RNase-like AVRA effectors unbound to receptors. AVRA13-1 interacts via its basal loops with MLA13 C-terminal leucine-rich repeats (LRRs) and the central winged helix domain (WHD). Co-expression of structure-guided MLA13 and AVRA13-1 substitution variants show that the receptor–effector interface plays an essential role in mediating immunity-associated plant cell death. Furthermore, by combining structural information from the MLA13–AVRA13-1 heterocomplex with sequence alignments of other MLA receptors, we engineered a single amino acid substitution in MLA7 that enables expanded effector detection of AVRA13-1 and the virulent variant AVRA13-V2. In contrast to the pentameric conformation of previously reported effector-activated CNL resistosomes, MLA13 was purified and resolved as a stable heterodimer from an in planta expression system. Our study suggests a common structural principle for RNase-like effector binding to MLAs and highlights the utility of structure-guided engineering of plant immune receptors for broadening their pathogen effector recognition capabilities. Synopsis: Mildew Locus A (MLA) proteins are plant immunoreceptors that bind strain-specific powdery mildew effectors encoded by fungal avirulence genes (AVRas). This study reports the cryo-EM structure of barley immunoreceptor MLA13 in complex with its cognate fungal effector AVRA13-1, revealing common structural principles. Cryo-EM analysis shows that AVRA13-1 adopts an RNase-like fold and forms a heterodimeric complex with MLA13. AVRA13-1 interacts with the C-terminal leucine-rich repeat and central winged helix domains of MLA13. A single amino acid substitution variant expands recognition capacity of the related MLA7 receptor for detection of AVRA13-1 and AVRA13-V2. Cryo-EM analysis shows the RNase-like fold of a fungal effector and how it complexes with its cognate nucleotide-binding leucine-rich repeat (NLR) receptor. [ABSTRACT FROM AUTHOR]